Without limiting the scope of the invention, its background is described in connection with cadmium sulfide layers and cadmium sulfide film formation reactions.
Efficient photovoltaic devices, such as solar cells, have been fabricated using absorber layers made with alloys containing elements of group IB, IIIA and VIA, e.g., alloys of copper with indium and/or gallium or aluminum and selenium and/or sulfur. One common combination of the aforementioned elements is copper-indium-gallium-diselenide (CIGS) and the resulting devices are often referred to as CIGS solar cells. The CIGS absorber layer may be deposited on a substrate.
Typical deposition techniques include evaporation, sputtering, chemical vapor deposition, and the like. These deposition processes are typically carried out at high temperatures and for extended times. Both factors can result in damage to the substrate upon which deposition is occurring. Such damage can arise directly from changes in the substrate material upon exposure to heat, and/or from undesirable chemical reactions driven by the heat of the deposition process. Thus, very robust substrate materials are typically required for fabrication of CIGS solar cells.
In CdTe and CIGS solar cells, cadmium sulfide works as a window layer that does not absorb much light, however, its thickness optimization is important to make high efficiency solar cells. Although solution growth of cadmium sulfide thin films has been implemented for many years for its simplicity and cost, it is a challenge currently in the art to control the thickness and density of the films because of the reaction complexity.
Generally, cadmium sulfide films are grown using cadmium complexing agents such as ammonium hydroxide, amines, and acetates; however, measuring cadmium concentration changes in these cadmium complexing compositions is impossible since the buffer cadmium concentration and cadmium ion concentration is below the detectable limit of the electrode.